Moisture in the Atmosphere

GEOG/ENST 2331 – Lecture 9 Ahrens: Chapter 4 Last lecture Atmospheric mechanics PGF, gravity, Coriolis effect, friction Geostrophic Cyclones and anticyclones Moisture in the Atmosphere Hydrologic cycle Changes of phase Adiabatic processes Humidity and comfort Hydrologic Cycle

Ahrens: Figure 5.1 Phases of water

Ahrens: Fig. 4.3 Changes of phase Evaporation Liquid molecules break free to become gaseous Surrounding material becomes cooler

Gaseous molecules pull together to form a liquid Surrounding material becomes warmer Saturation and equilibrium • Water exposed to a gas (or vacuum) will evaporate into it • Water vapour will bond together and condense • Saturation is the point at which the evaporation and condensation rates are equal

A&B: Figure 5-2 More changes of phase Freezing Melting

Sublimation Solid to gas Deposition Gas to solid Sometimes also called sublimation Ahrens: Fig. 2.3 Lecture outline Hydrologic cycle Humidity Absolute and specific Vapour Saturation Adiabatic processes Humidity and comfort Water vapour indices Absolute Humidity Indicates the density of expressed as a concentration (typically g/m3) Specific Humidity Represents a given mass of water vapor per mass of air as a ratio (typically g/kg) Vapour pressure The amount of pressure exerted by water molecules in the air (typically mb or hPa) Also called the partial pressure of water vapour Saturation vapour pressure

Water at equilibrium Dependent on temperature and pressure Increases non-linearly with temperature

Similar to saturation specific humidity Ahrens: Active Fig. 4.10 Relative Humidity Indicates the amount of water vapour in the air relative to the saturation point Vapour pressure RH = Saturation vapour pressure Specific humidity = Saturation specific humidity

Dependent on specific humidity, temperature, and pressure Ahrens: Active Fig. 4.11 Dew point temperature Another index of moisture (not temperature) Temperature at which the vapour pressure would equal the saturation vapour pressure High vapour pressure means high dew point

If the air cools to this temperature, it will become saturated

If T = Td then RH = 100% Dew Point Temperature A&B: Figure 5-9 Methods of Achieving Saturation Cool the air to the dew point

Add water vapour to the air

Mix cold air with warm air If the warm air has higher specific humidity Achieving saturation A&B: Figure 5-9 Dew At night the ground cools quickly Emission of IR

Air above the ground cools due to conduction Reaches dew point temperature Condensation begins Frost Like dew, but with a dew point temperature below 0°C (frost point temperature) Deposition into ice crystals rather than condensation

Frozen dew: dew point is above 0°C, but temperature drops below after condensation Lecture outline Hydrologic cycle Humidity Adiabatic processes Dry adiabatic cooling Humidity and comfort Temperature change Diabatic process: adds or removes energy from a system Heat transfers

: changes temperature without adding or removing heat Diabatic processes Mixing Warm, moist air moves over a cool surface Radiation Night-time cooling Adiabatic processes Changes in pressure cause changes in temperature If the pressure drops, a parcel of air will expand and cool down If the pressure rises, a parcel of air will be compressed and warm up What is a ‘parcel’ of air? A volume of air with distinct properties No fixed dimensions or shape Creates a mental picture for visualizing atmospheric conditions I talk about a cubic metre of air on occasion – a blob of air Dry adiabatic cooling

Parcels expand and cool at the dry adiabatic of 1oC/100 m (10°C/km) A&B: Figure 5-15 Adiabatic warming Descending air warms at exactly the same rate Ahrens: Figure 6.2 Lecture outline Hydrologic cycle Humidity Adiabatic processes Humidity and comfort Humidex It is not the heat, it is... the Humidity ‘Dry heat’ Sweat evaporates readily Body cools down easily High humidity Sweat does not evaporate Body cannot shed heat Even when it is cold? ‘Dry cold’ Sweat evaporates readily But the body does not sweat much High humidity Sweat condenses in clothing Damp clothing provides much less insulation Humidex

Ahrens: Fig. 4.19 Summary Three important humidity indices: Vapour pressure, relative humidity, dew point temperature Adiabatic processes Rising air expands and cools Sinking air is compressed and warms Humidex Interesting but not so important

Coming up Atmospheric stability Saturated lapse rates Ahrens: Chapter 6